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Losses in Cooling.                            547

are parallel hyperbolas at 320 and 201 respectively. In com-
pressing the air without subtracting heat, its temperature rises to
320, and the pressure curve is the adiabatic (''no heat passing
through') from A to B, the volume being now reduced to '5 with
pressure 45 Ibs.  Suppose the temperature next to lower to 60
during transit to motor, pressure remaining constant, which is
practically true, the volume will decrease from B to c, viz., to "32.
Next let the air expand behind the motor piston, without adding
heat, and its pressure will fall to 15 Ibs., while its volume becomes
74, and the expansion curve will be the adiabatic c D, the final
temperature of which is - 201. The area A B F G shews the work
given to the gas, c D G F that restored to the motor, and the loss
due to cooling is the area A BCD, being here about 27/0- In
practice the curves would more nearly approach the thick dotted
lines, but there are losses in steam cylinder, main, and motor,
which may reduce the efficiency to 30% instead of the 73/0
shewn. (See also fp. 773 and 881.)

Formerly simple steam-engines were employed as compressors,
but these are now replaced by compound engines; much im-
provement too, has been made in the methods of cooling. It
being granted that isothermal compression is the ideal condition,
the old water-jacketing proved inefficient, as removing the heat
after adiabatic compression had been permitted. Water pistons
were little better, being cumbrous and slow; while water spraying
in the air cylinders both spoilt the cylinders and gave but a slight
further advantage, for the time was too short for the heat
to be taken up. The greatest improvement was made by the
introduction of two-stage compression, or the performing of the
work in two cylinders, with an intermediate cooler. It can easily
be shewn that if a succession of such cylinders and intermediate
coolers be used, the compression may be truly isothermal, thus
gaining a large portion, but not all, of the lost work area, for loss
in the motor may still occur. The blocking of the ports with ice
or snow on account of the low "temperature of the motor exhaust
caused trouble ..Which an attempt to overcome was made in 1887,
by re-heating the air entering the motor, but .th<^ economic results
proved such a surprise, (:hat the method has ever since been
followed, with increasing success; and it is now clearly under-